Recently I read several manuscripts saying that limestones, consisting of CaCO3, can contain uranium (where Ca can be substituted with U) and thus can be used to directly measure radiometric age of sedimentary rocks.
Some review papers, for example, Rasbury and Cole (2009, Review of Geophysics; http://onlinelibrary.wiley.com/doi/10.1029/2007RG000246/full) shows that some examples of Devonian corals or Permian cements can give good U-Pb ages which are consistant with other methods.
If so, why don't we often use U-Pb age dating method for measuring radiometric ages of carbonates which cannot be measured otherwise, for example, those of Precambrian?
Dear Jeong-Hyun Lee,
U is not soluble in water's normal pH. But it is soluble in the form of uranyl carbonate UO2(CO3). It seems that Precambrian limestones were formed like the newer ones.
Anyway, it seems that in Precambrian limestone U em Pb are present. If so, the U-Pb dating may be used.
The problem with sediments is that if during diagenesis the sediment looses water it looses also some uranyl and, depending on the Pb present or not, the result will be a lie. I mean that U may be mobilized while Pb stays in the sediment. And therefor the age will be not true.
Another problem is that the age is not of the sedimentation but the age of the diagenesis what can happen long time after sedimentation.
In any case, U-Pb may be used for dating Precambrin and newer sediments but care must be taken when interpreting the results.
Dear Dr. Lee:
Have a look at the hexavalent and tetravalent ion of U and you will hardly see any chance for U to substitute for bivalent Ca in calcite at a considerable amount so as to be amenable for U-Pb dating. There are a few examples where U is contained in calcareous rocks. It is the calcrete-type:
• Uranium deposits cementing minerals
• Lithology is made up of fine-grained surficial sand and clay
• Origin:
Uranium-enriched basement granites were deeply weathered in a semi-arid to arid climate.
U is transported into valley-fill sediments along Tertiary drainage channels, and in playa lake sediments, where concentration took place primarily by evaporation
• Age: Surficial uranium deposits are broadly defined as Cenozoic near-surface uranium concentrations in semi-consolidated sediments and soils
• Mineralogy: carnotite, (meta-) tyuyamunite, francevillite, strelkinite, meta-autunite,
I do not see any realistic chance to solve this issue of age dating using U-Pb systems in carbonate. Another option is the group of accessory minerals in limestones which are rather seldom excluding apatite.
I wish you much success
H.G.Dill
Ms. Lee, regarding your question, I recommend you take a look at the work of our coleague Marly Babinski (on ResearchGate Marly Babinski). She has been working with radiometric dating of precambrian carbonates for some time.
Here is an important reference on the subject.
Article U/Pb dating of geodic calcite: new insights on Western Europ...
Cheers!
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